The U.S. government is threatening action against Chinese airlines, bullying Beijing to lift coronavirus restrictions on passenger flights.

United Airlines wants to resume four routes to China next month while Delta Air Lines wants to resume two. But a temporary Chinese policy allows an airline only one route.

“China has, over the objections of the U.S. government, impaired the operating rights of U.S. carriers,” Department of Transportation Assistant Secretary Joel Szabat wrote to Chinese airlines in a public letter.

LOS ANGELES, CA – APRIL 05: Capt. William Elder of the inaugural American Airlines non-stop flight … [+]to Shanghai holds the US and Chinese flags from the cockpit of the passenger jet during festivities at Los Angeles International Airport April 5, 2011 in Los Angeles, California. American Airlines launched a new daily non-stop flight between Los Angeles and Shanghai, the first between the cities served by a U.S. carrier. Regional airline American Eagle added nine new non-stop from Los Angeles to Albuquerque, Boise, El Paso, Houston, Oklahoma City, Phoenix, Salt Lake City, Sacramento and Tucson. (Photo by Kevork Djansezian/Getty Images)

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Szabat ordered seven Chinese airlines to submit their U.S. flight schedules by May 27. It is a banal procedure of publicly available information – take off time, aircraft type, codeshare – but Szabat said the U.S. would assess if those Chinese flights “may be contrary to applicable law or adversely affect the public interest.”

This dispute dates to March when China restricted airlines to having one passenger flight into the country, a limitation that gave China time to develop health protocols for screening international flights.

The policy was universal; Chinese airlines were also capped at one weekly passenger flight.

China’s local coronavirus infections were rapidly decreasing but imported cases were growing. Chinese airlines and airports implemented biosecurity policies faster and more extensively than U.S. peers, even though all three major U.S. airlines partner with Chinese carriers they can call on to share experiences and best practices.

WUHAN, April 8, 2020 — Crew members of flight MU2527 of China Eastern airlines pose for group … [+]photos before their boarding at the Tianhe International Airport in Wuhan, central China’s Hubei Province, April 8, 2020. Wuhan on Wednesday lifted outbound travel restrictions, after almost 11 weeks of lockdown to stem the spread of COVID-19. (Photo by Cheng Min/Xinhua via Getty) (Xinhua/Cheng Min via Getty Images)

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The U.S. argues China’s flight restriction “imposes capacity limitations” that are “inconsistent” with their historical aviation treaty. However, the four page U.S. letter makes no reference to coronavirus or COVID-19, the extenuating circumstances behind the restrictions.

Chinese airlines are understood to want the policy lifted as well, believing they can safely transport passengers across more than one weekly flight.

Szabat said U.S. airlines should be able to “exercise their full bilateral rights.” The U.S. however has effectively limited airlines elsewhere, such as by prohibiting the entry of Europeans.

DALLAS, TEXAS – MARCH 20: American Airlines fleet services employees prepare to load cargo pallets … [+]on a 777-300 at Dallas/Fort Worth International Airport (DFW) bound for Frankfurt Airport in Germany during the cornoavirus (COVID-19) pandemic on March 20, 2020 in Dallas, Texas. The first cargo-only flight departs from Dallas Fort Worth International Airport (DFW) tomorrow, March 20, landing at Frankfurt Airport (FRA) March 21. The Boeing 777-300 will operate two round trips between DFW and FRA over the course of four days, carrying only cargo and necessary flight personnel. This is the first scheduled cargo-only fight since 1984 when American retired the last of its Boeing 747 freighters. (Photo by Tom Pennington/Getty Images)

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In its complaint about passenger flights, the U.S. did not acknowledge China helped all three major U.S. airlines start and increase cargo flights.

Cargo airlines and ad hoc providers – including a 767 from the owner of the New England Patriots – have also received assistance for China-U.S. cargo flights, carrying medical supplies and general freight.

BOSTON, MA – APRIL 2: A New England Patriots jet arrives at Logan Airport in East Boston on Apr. 1, … [+]2020 after flying from China with a massive shipment of over one million N95 masks which will be used in Boston and New York to help fight the spread of the coronavirus. Part of the shipment is driven into a hanger after being off loaded from the plane. (Photo by Jim Davis/The Boston Globe via Getty Images)

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China’s one weekly flight policy, announced March 26, had a further clause that future flights could not exceed the levels of March 12. U.S. airlines started pulling down their China flights in late January and had no services by March 12, making them ineligible for even one weekly flight.

The March 12 baseline may soon be removed, China told the U.S. on May 14.

“However, the restriction to once-weekly service on one route to China would remain in place,” Szabat wrote. That would preclude Delta and United from operating the multiple flights they want across numerous routes.

Delta and United submitted passenger flight applications to China but have not received a response, Szabat said.

Delta wants to operate two routes: Detroit-Shanghai and Seattle-Shanghai. United told employees it intends to fly four routes: Chicago-Shanghai, Newark-Shanghai, San Francisco-Beijing and San Francisco-Shanghai.

All flights would be daily. Delta’s proposed 14 weekly passengers flights and United’s proposed 28 weekly flights exceed the current limit of one for each airline.

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Airlines from Qatar, South Korea and Turkey also plan to serve multiple Chinese destinations from June, which would require a lifting of the restriction, known as the “Five Ones.” That limits one airline to having one flight a week on one route from one Chinese city to one foreign city.

The timing of the U.S. is questionable, applying pressure right after China started the so-called “Two Sessions,” its most important political event. Public policy proclamations decrease in the lead up to the Two Sessions, where the government reviews past work and sets out broad objectives.

The gathering is so high profile that Zhenglin Feng, the administrator of aviation body CAAC, visited Beijing’s Capital and Daxing airports to ensure preparations and see their latest coronavirus prevention initiatives, such as increased terminal ventilation – which airports globally have shunned in favor of centralized air conditioning and heating.